The present invention relates to a plasticizing device in an injection molding machine and, more particularly, to an in-line type screw of the plasticizing device.
FIG. 3 schematically shows a conventional screw 11, in particular the portion around a resin supply port 13.
The operation of the conventional in-line screw 11 relies upon frictional force alone so that it could never stably plasticize the material. In particular, platicization is often rendered unstable when a lubricating oil is mixed in the resin material for the purpose of improving lubrication of the molded product.
Unstable plasticization is caused when the resin material 4 cannot smoothly be fed forward due to clinging of the resin on the screw 11. This problem takes place when the friction between the resin 4 and the surface 11b of the screw 11 is not substantially different from the friction between the resin 4 and the inner surface 12a of the cylinder.
Good plasticizing effect is obtained when the friction between the resin 4 and the inner surface 12a of the cylinder is considerably higher than that between the resin 4 and the screw surface 11b. In order to realize this condition, attempts have been made such as roughening of the inner surface 12a of the cylinder and provision of axial grooves in the inner surface 12a of the cylinder.
These attempts such as roughening or grooving of the cylinder inner surface 12a cause problems such as an increase in the driving torque for driving the screw 11 or imperfect plasticization particularly when the speed of the screw is increased.
Thus, no reliable method for stabilizing plasticization of resin has been proposed.
Accordingly, an object of the present invention is to provide an improved screw for injection molding which can overcome the above-described problems of the prior art.
To this end, according to the present invention, there is provided a screw of a screw-in-line type plasticizing device, characterized in that a portion of a screw flight facing a resin supply port has a parallelogrammic axial section which is inclined towards the discharge end of the screw.